In its broad aspects, the present invention is directed to secondary electrical energy storage systems of the aqueous type. A metal halogen hydrate electrical energy storage system of the type to which the present invention is applicable is fully described in U.S. Pat. No. 3,713,888, entitled "Halogen Hydrates", issued Jan. 30, 1973. This patent is owned by the same assignee as the present invention and details thereof beyond those herein described are incorporated in this application by reference. Metal halogen hydrate electrical energy storage systems or secondary storage batteries are conveniently categorized as being of the high energy density (H.E.D.) type because of their capacity to supply upwards of 50 watt hours of electric power per pound of weight. This high electrical energy capacity coupled with the compactness and low weight of such secondary storage batteries has rendered them particularly satisfactory for use as principal and auxiliary sources of electrical energy in either mobile (electric vehicles) or stationary (utility load leveling) power plant systems.
The present invention pertains primarily to zinc halogen battery systems, and more particularly to zinc chlorine battery systems, although it should be appreciated that the invention described herein may be equally applicable to other metal halogen battery systems. The chemical reactions which occur in a zinc chlorine hydrate battery are relatively straightforward. During charge, the electrolyte (a solution of zinc chloride in water) is flowed through the battery with the aid of a circulator. As electrical direct current is passed through the battery from an external source, zinc metal is electro-deposited on the negative electrode (typically relatively dense graphite) of the battery as a uniform, non-porous solid. Simultaneously, chlorine gas, generated at the positive electrode (typically porous graphite or ruthenia-catalyzed porous titanium) is carried away with the circulating electrolyte stream. Outside of the battery, the chlorine gas is admixed with cold water and a pale yellow solid called chlorine hydrate is formed. The solid chlorine hydrate (Cl.sub.2.xH.sub.2 O) is retained separate from the battery. During discharge, the aqueous zinc chloride electrolyte is again circulated through the battery thereby carrying chlorine, which is slightly soluble in the electrolyte, to the chlorine electrode of the battery and permitting current to be withdrawn from the battery. To replace the chlorine in the electrolyte, the chlorine hydrate is heated in a controlled manner to release chlorine from the hydrate.
It has been found that a fundamental problem with zinc electrodes in either alkaline or acid solutions is that the zinc tends to form dendritic or nodular growths during the deposition process. Eventually, such dendritic growths lead to internal shorting of the battery, thereby limiting the charge capacity and decreasing the electrochemical energy efficiency of the battery. One solution which has been proposed to the problem of dendrite formation is the use of additives in the electrolyte solution; however, this additive approach has not met with total success. One may not be able to use the organic additives that have previously been used in the electrodeposition of metals, such as zinc, because the chlorine decomposes the organic additives. Further, the additives, through various mechanisms, may cause unwanted electrode polarizations. It has also been found that some additives tend to precipitate or salt out during repeated recharing; examples of such additives being benzotriazole, benzene sulfonamide, toluene sulfonamide, chlorotoluene sulfonamide and thiourea. Lastly, attempts at using certain organic additives, such as those described in U.S. Pat. No. 3,793,079 and U.S. Pat. No. 3,811,946 both owned by the assignee of the present invention, although effective, have not met with total success in all cases, since they sometimes tend to degrade over extended periods.
It has generally been found that the use of an inorganic additive such as thallium provides good leveling and battery capacity. However, the amount of thallium required to be effective is relatively large and may cause some unwanted voltaic polarizations. In addition, thallium is highly toxic. Therefore, it is still highly desirable to have some means of controlling the morphology of the zinc deposit, thereby increasing charge capacity of the battery.